1. Field of the Disclosure
The present invention relates to the field of digital subscriber line (DSL) technology, and more particularly to a method, system, and device for crosstalk cancellation of multi-pair xDSL.
2. Discussion of the Related Art
A pass-band transmission xDSL adopts discrete multi-tone modulation (DMT) technology for modulation and demodulation. A system for providing multiple DSL access is referred to as a digital subscriber line access multiplexer (DSLAM), a connection relation of which is shown in FIG. 1. The subscriber end xDSL transceiver 120 includes a subscriber end transceiver unit 121 and a splitter/integrator 122. In an uplink direction, the subscriber end transceiver unit 121 receives and amplifies a DSL signal from a computer 110, and sends the amplified DSL signal to the splitter/integrator 122. The splitter/integrator 122 integrates the DSL signal from the subscriber end transceiver unit 121 with a plain old telephone service (POTS) signal from a telephone terminal 130. The integrated signal is transmitted through multiple unshielded twisted pairs (UTPs) 140 and received by a splitter/integrator 151 in a center office end xDSL transceiver 150. The splitter/integrator 151 splits the received signal, sends the POTS signal to a public switched telephone network (PSTN) 160 and sends the DSL signal to a center office end transceiver unit 152 of the center office end xDSL transceiver 150. The center office end transceiver unit 152 re-amplifies the received xDSL signal and then sends it to a network management system (NMS) 170. In a downlink direction, signal is transmitted in a sequence reverse to the above processes.
As a frequency band adopted in the xDSL technology is continuously increased, the crosstalk becomes increasingly severe, especially in high frequency bands. Referring to FIG. 2, because uplink and downlink channels of the xDSL adopt frequency division multiplexing technology, a near-end crosstalk (NEXT) does not cause significant influences to the system performance; however, a far-end crosstalk (FEXT) brings severe impacts on the transmission performance of the lines. When xDSL services are activated in a bundle of cables upon being requested by a plurality of subscribers, certain lines may suffer from a low transmission rate and an instability problem; even the xDSL services may not be activated due to FEXT, which results in a low line activation rate of the DSLAM. For example, according to current technical standards for xDSL, theoretically, VDSL2 (vectored-DSL) can provide an uplink-downlink symmetrical rate of up to 100 Mbps. However, an obvious problem may occur during the actual deployment due to FEXT and high frequency signal attenuation.
Currently, a vectored-DSL technology has been proposed in the industry, which mainly uses the DSLAM terminals to perform joint transmitting and receiving, so as to cancel the interference of FEXT by means of signal processing, thereby eventually enabling each signal to be free of FEXT interference.
FIG. 3 shows a situation where a center office end jointly sends and subscriber ends respectively receive downlink vectors. The process of receiving downlink vectors is described as follows.                1. A matrix HT is expressed as HT=Qi·Ri according to QR decomposition. Herein, R is an upper triangular matrix; Q* is a unitary matrix, i.e., QQ*=Q*Q=1, in which the superscript * represents a conjugate transpose; HT is a transpose matrix of H. Accordingly, H=RTQT.        
2. It is assumed that xi=QiT*xi′, and xi′=Ri−T diag(RiT){tilde over (x)}i, in which diag represents a diagonalizable matrix.
If yi=Hixi+Ni=RiTQiTQiT*Ri−T diag(RiT){tilde over (x)}i+Ni=diag(RiT){tilde over (x)}i+Ni, as for a noiseless channel, an output is turned to be ŷ=diag(RiT){tilde over (x)}i, which is a diagonal matrix, thereby canceling the crosstalk.
FIG. 4 shows a situation where the subscriber ends respectively send and the center office end jointly receives uplink vectors. The process of receiving uplink vectors is described as follows.                1. The matrix H is expressed as Hi=Qi·Ri according to QR decomposition. Herein, R is an upper triangular matrix; Q is a unitary matrix, i.e., QQ*=Q*Q=1, in which the superscript * represents a conjugate transpose.        2. An uplink receiving end is:Yi=Hixi+Ni  (1),        
Both sides of Equation (1) are multiplied by Q*, so as to obtain the following equation:Ŷi=Q*(Hixi+Ni)  (2).Accordingly,Ŷi=Q*·Q·Rixi+Q*·Ni=Rixi+Q*·N  (3).
As seen from Equation (3), as for a noiseless channel, an output is Ŷi=Rixi,1≦i≦L, which is an upper triangular matrix.                3. An output value is estimated through using generalized decision feedback equalization (GDFE).        
It can be seen that, the Lth output is a value without crosstalk and can be estimated by using a simple decoder, so as to obtain the Lth output value. By means of subtracting the Lth estimated result from the (L−1)th output, the crosstalk of the (L−1)th tone caused by the Lth tone is cancelled. Through simple estimation, the (L−1)th output value can be obtained, and so forth. Therefore, the first output value is obtained by subtracting the previously estimated value, and the ISI (Inter Symbol Interference) is thus cancelled.
The shared channel H in FIGS. 3 and 4 may be expressed as a matrix:
H(f)=[Hkm(f)]k=1 . . . L,m=1 . . . L, in which Hkm (f) is a propagation equation from a pair m to a pair k. In practice, k is equal to m and both pairs are equal to the number of channels involved in a crosstalk effect on each other in the shared channel, which is set as L herein. Thus, H is a L×L channel transmission matrix. A processor processes the L×L channel transmission matrix, so as to cancel the interference of FEXT.
A typical DSL bundle generally consists of 50 to 100 twisted pairs. If it intends to cancel all of the crosstalk, the processor generally needs to process a H matrix of 50×50 or 100×100, which exceeds the current computation complexity constraints of digital signal processing at the centre office (CO) end.